A thermal-type air flow meter that measures a gas flow rate is configured to be provided with a flow rate detection unit to measure a flow rate, and perform heat transfer between the flow rate detection unit and a gas as a measurement object so as to measure the gas flow rate. The flow rate to be measured by the thermal-type air flow meter is widely used as an important control parameter in various types of devices. A characteristic of the thermal-type air flow meter is that it is possible to measure the gas flow rate, for example, a mass flow rate with a relatively high accuracy as compared to a flow meter of another type.
However, there is a demand for further improvement of measurement accuracy of the gas flow rate. For example, there is an extremely strong demand for fuel saving or exhaust gas purification in vehicles to which internal combustion engines are mounted. In order to cope with such demands, measurement of the amount of intake air, which is a key parameter of the internal combustion engine, with high accuracy has been required. The thermal-type air flow meter that measures the amount of intake air to be guided in the internal combustion engine is provided with a secondary passage which takes a part of the amount of intake air, and a flow rate detection unit arranged in the secondary passage, measures a state of gas to be measured which flows in the secondary passage by the flow rate detection unit performing heat transfer with respect to the gas to be measured, and outputs an electrical signal indicating the amount of intake air to be guided in the internal combustion engine. Such a technique is disclosed in Japanese Patent Application Laid-Open No. 2011-252796 (PTL 1), for example.
An air flow rate measuring device of PTL 1 is provided with a housing forming an internal flow passage in which intake air flows, a sensor chip arranged inside a secondary flow passage, and a circuit chip processing an electrical signal generated by the sensor chip. The sensor chip and the circuit chip are assembled as a sensor assembly serving as a single component (paragraphs 0027 and 0031). The housing has a hole in which the sensor assembly is fit, and a surface in which the hole is formed has two contact surfaces which are in surface contact respectively with two contact surfaces provided in the sensor assembly (paragraph 0033). The surface in which the hole is formed has a rib in surface portions except for the two contact surfaces, and the sensor assembly is fixed to the housing by being press-fitted in the hole so as to be in press contact with a leading end of the rib (paragraph 0034). Accordingly, transmission of linear expansion between the sensor assembly and the housing is blocked such that a stress caused by a linear expansion difference is not applied to the sensor chip or the circuit chip, thereby suppressing a change in resistance value of elements such as the sensor chip and the circuit chip (summary).